Page 106 - Inorganic Mass Spectrometry - Fundamentals and Applications

96 Olesik

than a critical value are accelerated far enough away from the center of the
quadrupole during a half-cycle of the ac voltage to hit the rods and be lost. Ions
that have a masslcharge ratio greater than a critical value have a stable path
through the quadrupole. As a result, the rods in the x-z plane, as defined in Fig.
3.13a, act as a high-pass filter.
Remember that ions with a large mass/charge ratio are affected more by the
average (dc) voltage than by the rapidly changing ac voltage. If the average (dc)
voltage on the rods is negative 0-2 plane, as defined in Fig. 3.13a), ions with a
masslcharge ratio greater than a critical value are attracted to the rods until they
collide with the rods and are lost. Light ions follow the rapidly changing ac
potential so they are refocused toward the center of the quadrupole during the
portion of the waveform when the ac potential is more positive than the dc
potential, As a result, the rods in the y-z plane, as defined in Fig. 3.13a, act as
a low-pass filter.
in
Only ions that have a stable path both the x-z and y-z planes pass through
the quadrupole. The result is a bandpass filter that allows only ions with mass/
charge ratios within a narrow range to pass through the quadrupole and be
detected. The range of masslchar~e ratio ions that pass through the qua~pole
(the resolution of the quadrupole) depends on the ratio of the dc to ac amplitude,
the ac frequency, and the quad~pole radius.
"he masslcharge ratio ions that are stable in both the x-z and y-z planes can
be described in terms of a stability diagram (Fig. 3.13b) with variables
a and q. As
shown in Fig. 3.13b, depends on the magnitude of the dc voltage and q depends
a
on the amplitude of the ac voltage. Both a and q also depend on the masslcharge
ratio of the ion, the radius of the quadrupole, and the frequency of the ac voltage
applied to the rods. The masslcharge ratio for ions that pass through the quadru-
pole is varied by changing the dc voltage magnitude and the ac ampli~de while
maintaining a constant ratio (as shown by the mass scan line in Fig. 3.13b).
Sector inst~ments for ICP-MS are commercially available. These instru-
ments can provide either high resolution, to overcome spectral overlaps requir-
ing a resolution of about 9000 or less, or high sensitivity when used in a low-
resolution mode [ 127,1281. Because the main source of signal ~uc~ations are the
sample introduction system and plasma ion source themselves, simultaneous,
multicollector sector mass Spectrometers can provide high precision ratios, ap-
proaching the precision of thermal ionization mass spectrome~. Typically, multi-
collector instruments cannot provide resolution as high as single-detector sector
based inst~ments can. Multicollector sector mass spectrometers are discussed in
of
detail in Chapter 8. Recently, an instrument that consists a hexapole collision-
reaction cell and a single-sector mass spectrometer has been introduced. "he
collision-reaction cell is used to reduce the spread ion kinetic energies so that an
of
electric sector for energy filtering is unnecessary.
Time-of-flight (TOF) ICP-MS instruments are also now commercially